1. Field of the Invention
This invention relates to an optical device suitable for use with an optical amplifier, for example, of an optical communication system.
2. Description of the Related Art
In optical communications, an erbium-doped optical fiber amplifier which operates with a lowest loss wavelength (1.55 .mu.m) of an optical fiber of quartz is usually used as measures for solving the problem of the limitation to the loss of a transmission system and assuring a large amount of information.
FIG. 25 is a block diagram showing a construction of an erbium doped optical fiber amplifier 100 of the type described above. Referring to FIG. 25, the erbium doped optical fiber amplifier 100 shown includes an erbium doped fiber 102 for amplifying an optical signal, a front optical circuit 101 provided at a preceding stage to the erbium doped fiber 102, and a rear optical circuit 103 provided at a following stage to the erbium doped fiber 102.
The front optical circuit 101 includes a coupler (CPL) 101A, a photodiode (PD) 101B and an isolator (ISO) 101C. The photodiode 101B monitors an input optical signal to the front optical circuit 101 through the coupler 101A, and the isolator 101C prevents the resonance by reflection of light amplified by the erbium doped fiber 102. An output optical signal of the isolator 101C is inputted to the erbium doped fiber 102.
The rear optical circuit 103 includes an optical wave multiplexing-demultiplexing coupler (WDM coupler) 103A, a polarized light separator (PBS) 103B, a total reflection optical film (HR) 103C, an isolator (ISO) 103D, a pair of couplers (CPL) 103E and 103G, and a pair of photodiodes (PD) 103F and 103H. pump light inputted from a pair of laser diodes (LD) 107 and 108 is supplied through the WDM coupler 103A to the erbium doped fiber 102. On the other hand, an optical signal amplified by the erbium doped fiber 102 can be monitored by the photodiodes 103F and 103H. The isolator 103D prevents the resonance by reflection of light amplified by the erbium doped fiber 102 similarly to the isolator 101C.
The erbium doped optical fiber amplifier 100 further includes an alarm-monitor detection circuit 104, an automatic level controller (ATC circuit) 105, and an automatic level controller (ALC circuit) 106.
In the erbium doped optical fiber amplifier 100 shown in FIG. 25 and having the construction described above, an optical signal is amplified by the erbium doped fiber 102. In this instance, the input light is monitored by the photodiode 101B of the front optical circuit 101, and the amplified light is monitored by the photodiodes 103F and 103H of the rear optical circuit 103. In response to results of the monitoring, optical amplification control by the alarm-monitor detection circuit 104, the ATC circuit 105 and the ALC circuit 106 is performed.
By the way, in the erbium doped optical fiber amplifier 100 described above, a large number of optical elements or devices such as an input light level monitor, an output light level monitor, a pumping optical wave multiplexer and a spontaneous emission light removing band-pass filter (not shown) are required at the preceding and following stages to the erbium doped fiber 102. Further, in the erbium doped optical fiber amplifier 100, those optical devices are individually connected in multiple connection. Where such optical devices are individually connected in multiple connection in this manner, there is a subject to be solved in that a large mounting space and a great number of steps of fiber connecting operations are required. It is another subject to be solved that, since the individual optical devices are expensive, it is difficult to achieve miniaturization and reduction in cost of the product.
FIG. 26 is a block diagram showing a construction of an improved rear optical circuit 203 having a similar function to that of the rear optical circuit 103 of the erbium doped optical fiber amplifier 100. Referring to FIG. 26, the rear optical circuit 203 includes a plurality of dielectric multi-layer films interposed in a collimate beam system based on a single lens system so that it has a plurality of functions. Where the rear optical circuit 203 is incorporated in the erbium doped optical fiber amplifier 100, the mounting space and the number of steps of fiber connecting operations can be reduced.
It is to be noted that the rear optical circuit 203 shown in FIG. 26 includes an optical wave multiplexing coupler (WDM coupler) 203A, a polarized light separator (PBS) 203B, a pair of total reflection optical films (HR) 203C and 203G, an isolator (ISO) 203D, a coupler (CPL) 203E, and a pair of photodiodes (PD) 203F and 203H.
However, since the dielectric multi-layer film used to form the rear optical circuit 203 is expensive and the rear optical circuit 203 requires a plurality of such rear optical circuit 203, there is a subject to be solved that it is difficult to reduce the cost of the rear optical circuit 203.